All vehicles with enclosed cabs will at times have problems with the windshield fogging. This is caused by a natural build up of humidity from respiration which condenses on cold glass surfaces. Typically, this fog (which may also freeze) is removed from the windshield with air that has been conditioned to be dryer and warmer than the air in the cabin. In most applications, this air is delivered to the glass through a duct with louvered opening. Some current production tractors have HVAC units in the cab roof. In such tractors, the defogging air is directed downward and at the windshield at an angle. Because of this angle and because warm air tends to rise, the conditioned air can bounce off the windshield. This results in small cleared spots which then grow in size, and can result in the bottom portion of the windshield remaining fogged. It is desired to improve the speed and thoroughness of clearing the windshield of defog and/or frost.